This invention relates in general to printing and, more particularly, to an apparatus for supplying printing presses with a fountain solution.
In lithography, the printing process used in most current printing, particularly full color printing, a plate carries an etched image that is ink receptive, and the ink on this image is transferred to a surface that may be on paper or some other material, thereby imparting the image to the surface. A fountain solution, which is primarily water, settles over the blank areas of the plate and renders those areas ink-repellant. In the typical multicolor printing press the plate is normally on a rotating plate cylinder and indeed forms the cylindrical outside surface of that cylinder. The plate cylinder moves against an ink roller where it picks up both the ink and the fountain solution, the latter having been delivered to the ink roller in the ink train. The ink and fountain solution separate on the plate cylinder, the former going to the etched image on that plate and the latter to the blank areas. The plate cylinder may upon further rotation apply the coating of ink directly to the surface that is to be printed, but it usually runs against another cylinder, called a blanket cylinder, which picks up the inked image and then transfers it to the surface that is to be printed.
While the fountain solution is basically water, it does contain additives such as gum arabic to retain the water on the blank areas of the plate cylinder, an antioxidant to inhibit corrosion of the plate cylinder, antifungal agents to deter a film from developing in the fountain solution, at least during warm weather, and a release agent to prevent the paper or other material from sticking to the blanket cylinder.
Each press has a circulating tank or reservoir for holding the fountain solution, and the solution is withdrawn from the tank and introduced into the ink train, from which some of it passes onto the plate cylinder. However, much more is separated from the ink and returned to the circulating tank to cool. Of course, some of the fountain solution is lost with each revolution of the plate cylinder so the solution in the circulating tank for each press must be continually replenished. To this end the circulating tanks are provided with mechanical float-type valves to which a pressurized distribution line containing fountain solution is connected.
Systems have been developed for supplying a properly mixed fountain solution to the circulating tanks of presses automatically, but these systems are less than satisfactory. In the first place, they are quite expensive. Secondly, they operate on a metering principle, with the amount of additive that is introduced into water being determined by the electrical conductivity of the solution that is produced. This may produce misleading results where more than one chemical is added, particularly where each has some effect on the electrical conductivity of the solution. Furthermore, it is difficult to monitor the metering system, so one never knows whether or not the fountain solution has its additives proportioned correctly. Finally, a metering system, by reason of the fact that it injects the chemicals directly into a water line, sometimes back flows, notwithstanding the presence of check valves, and this may cause additives to enter the water supply for the building in which the printing operation is conducted.